The Monday 08 Sep 2014 à 16:29:26 (+0200), Paolo Bonzini wrote : > Il 08/09/2014 14:18, Benoît Canet ha scritto: > > The algorithm used was defined on the list while discussing the new IO > > accounting > > overhaul. > > See http://lists.nongnu.org/archive/html/qemu-devel/2014-08/msg04954.html > > > > Also the module takes care of computing minimal and maximal values over the > > time > > slice duration. > > > > Signed-off-by: Benoît Canet <benoit.ca...@nodalink.com> > > If you add > > int64_t cpu_get_clock(void) > { > return my_clock_value; > } > > to the test, and use a QEMU_CLOCK_VIRTUAL-based average, you should be > able to advance the clock directly in the test with no need for sleep() > and with 100% deterministic results. > > > > > +/* Check if the ta->periods seconds time slice has expired > > + * > > + * If the slice has expired the counters will be reseted > > + * > > + * @ta: the timed average structure used > > + */ > > +static void timed_average_check_expiration(TimedAverage *ta) > > +{ > > + int64_t now = qemu_clock_get_ns(ta->clock_type); > > + > > + /* if we are still in the period slice do nothing */ > > + if (now < ta->expiration) { > > + return; > > + } > > + > > + /* the slice has expired -> create a new slice */ > > + ta->min = UINT64_MAX; > > + ta->sum = 0; > > + ta->max = 0; > > + ta->count = 0; > > + timed_average_set_expiration(ta); > > +} > Thanks for reviewing.
> This can produce very noisy results if you invoke min/avg/max at the > wrong time. Some alternatives include: > > - create two windows, with twice the suggested expiration period, and > return min/avg/max from the oldest window. Example > > t=0 |t=1 |t=2 |t=3 |t=4 > wnd0: [0,1) |wnd0: [1,3) | |wnd0: [3,5) | > wnd1: [0,2) | |wnd1: [2,4) | | > > Values are returned from: > > wnd0---------|wnd1---------|wnd0---------|wnd1---------| This is neat. As I think that knowing the minimal and maximal latencies of a block device would be handy I will implement this. Best regards Benoît > > > - if you do not need min/max, you can use exponential smoothing, with a > weighted factor that depends on the time since the last sample. > http://www.drdobbs.com/tools/discontiguous-exponential-averaging/184410671 > -- for example, giving 90% weight to the last second. Of course the > exponential nature means that, in that case, 1-sqrt(10%)=68.3% weight is > given to the last half second, 21.6% weight is given to the previous > half second, and 10% to the entire previous history. This cannot give > min/max, but can give avg/stdev. > > Paolo >
